Several unusual in vivo murine lymphoma lines were established and studied for surface and functional characteristics. The lines were derived from AKR/J strain mice and are noteworthy in that they exhibit characteristics of both T and B cells. When passaged into young normal AKR/J hosts, the splenic cells appear first like T-cell lymphomas and then acquire aspects of both T and B cells. Characteristics studied were Thy 1 antigen, Ly antigen, Ia antigen, Fc receptor, complement receptor, mitogen responsiveness, and surface immunoglobulin. The Thy plus tumor cells in lymph node remain Ig negative. It is possible, 1) that tumor cells differentiate to Thy plus Ig plus cells in spleen, but not in lymph node or 2) that two tumor subpopulations exist, one of which grows in spleen and lymph nodes (Ig minus), while the other, (Ig plus) grows more slowly and selectively in spleen. Attempts are being made to determine whether these tumor cells are differentiating, and if so, what the factors are which control this differentiation. BIBLIOGRAPHIC REFERENCES: Greenberg, R. S., Mathieson, B. J., Campbell, P. S. and Zatz, M. M.: Multiple occurrence of spontaneous AKR/J lymphomas with T and B-cell characteristics, J. Immunol. 118: 1181-1190, 1977. Hurwitz, E., Zatz, M. M. and Segal, D.: The biological activity of affinity cross-linked oligomers of rabbit immunoglobulin G as tested by antibody dependent cell-mediated cytolysis (ADCC). J. Immunol. 118: 1348-1353, 1977.